Mine is the microphone from my voice control module I installed on my AVIC-N2 headunit.
However in my picture, I broke the top clip off the A-pillar, so it's sagging and looks pretty bad, hah. It's usuallly tucked neatly against the side.
My guess is it's the microphone from where your car was wired for a car phone a while back. (my dad's old jeep had the same kind of thing when he had a car phone years back)